High power pulsed fiber lasers are currently in demand for a number of applications and uses. For example, numerous material processing applications such as memory repair, milling, micro-fabrication, drilling, etc. require pulsed laser systems which provide, among other requirements, high pulse energy, excellent pulse amplitude stability and beam quality, narrow linewidth and great flexibility in terms of control of the pulse amplitude and temporal duration, such as pulse to pulse control over the temporal duration at high (>100 kHz) repetition rates. Pulse bursting with in-burst pulse rate >100 MHz is also desirable.
The temporal duration of laser pulses is an important parameter for the optimization of laser material processing procedures, as for example explained in U.S. patent application no 2007/0272668 (ALBELO et al). Laser pulses in the millisecond to nanosecond regime are used to machine several types of material, providing high processing speed. However, such long pulses can cause heating, melting and vaporization of the target material, which leads to undesired thermal side-effects such as low quality of the machined surface and micro-cracks. It is known in that art that a limited heat-affected zone on the processed material and little or no collateral damage is typically obtained from cold ablation performed with femtosecond optical pulses. However, femtosecond lasers are often complex and expensive. In addition, the ablation process is inherently slow, since the layer which is removed is usually very thin compared to that obtained using thermal ablation with longer pulses.
Picosecond laser pulses are increasingly gaining attention in the industry. The time scale involved in such processes combines the benefits of light-matter interaction dynamics at both femtosecond and nanosecond regimes. For instance, laser pulse energy on a work surface may be increased above the cold ablation threshold while providing a higher speed process than femtosecond pulses. Trains of picosecond laser pulses emitted at high repetition rates in a burst regime are also very interesting to the industry (see for example P. Forrester et al., “Effects of heat transfer and energy absorption in the ablation of biological tissues by pulse train-burst (>100 MHz) ultrafast laser processing”, Proc. of SPIE Vol. 6343, 63430J (2006); P. Forrester, et al., “Generation of tailored picosecond-pulse-trains for micro-machining”, Proc. of SPIE Vol. 6108, 6108-37 (2006); and U.S. Pat. No. 6,552,301, issued Apr. 22, 2003 to HERMAN et al.). Under such conditions, the time interval between successive pulses is short enough for heat to accumulate at the work surface, thus conditioning the material for subsequent ablation by multiphoton ionization with high laser beam intensities. This ensures a clean ablation with smooth features.
The cold ablation threshold may vary significantly depending on several parameters, such as the material being processed, the pulse repetition rate, the pulse duration and the laser wavelength. Also, the quality of the laser machined surface and the machining speed are strongly dependent on these parameters. For these reasons, the optimization of the machining process for a specific material can greatly benefit from laser architectures that provide control on the pulse repetition rate, amplitude and duration, in simple pulse or burst regimes. Furthermore, the ability to adjust the pulse duration and amplitude from pulse to pulse provides further flexibility for process speed and quality optimization. For instance, the same laser could be used with long pulses to thermally ablate large amounts of materials, followed by short pulses or pulse bursts that would provide a better quality of machined surface.
It is also of interest that picosecond pulses are characterized by a high peak power (tens to hundreds of kilowatts for micro-joules pulses) and a narrow linewidth (less than 1 nm for transform limited pulses). This combination is very advantageous for frequency conversion (second, third and fourth harmonic), which opens up significantly the range of applications a single powerful picosecond source can address.
Mode-locked femtosecond laser, bulk or fiber-based, can be modified to produce picosecond pulses. Generally speaking, mode-locked fiber lasers are considered particularly attractive for ultra-short pulse generation, via either passive or active mode-locking. The pulse-generation mechanism in such lasers depends on the physic of the cavity. Known cavity configurations include linear cavities, ring lasers and figure-of-eight cavities. To produce picosecond pulses in such a mode-locked regime, a narrow spectral filter placed inside the laser cavity controls the duration of the pulses by the virtue of the Fourier transform. Usually, such designs suffer from a lack of flexibility since they require a tuning of the filter bandwidth to change the pulse duration. This tuning can necessitate moving parts.
Mode-locked lasers can also be adapted for producing picosecond pulse trains combined with a slicer or pulse picker, which selects the pulses that constitute the “burst”. Actively mode-locked fiber lasers allow for the generation of picosecond pulses at high repetition rates, such as for example shown in U.S. Pat. No. 6,108,465 (IIDA et al.) and U.S. patent application published under no. 2006/0245456 (LASRI et al.). However, the timing between successive pulses cannot be adjusted arbitrarily; it is rather determined by the harmonics of the laser cavity and the fundamental pulse repetition frequency. Additionally, tunable pulse durations of a few tens of picoseconds are difficult to obtain, since complex pulse shaping mechanisms occur along the pulse propagation within the fiber laser cavity.
Picosecond pulses can also be produced with gain-switched semiconductor laser diodes, where pulses are advantageously generated on demand by an electrical drive pulse. However there is usually little correlation between the temporal characteristics of the electrical drive pulse and the corresponding optical pulse. The optical pulse is in fact the impulse response of the device, and therefore has a duration which differs from chip to chip. In addition, such diodes offer very little control on the amplitude of the emitted pulses, which are often followed by relaxation oscillations.
Solid-state gain media may be used for high repetition rate ultrashort (e.g. picosecond or less) pulse lasers (see for example U.S. Pat. No. 6,778,565 (SPUEHLER et al.) U.S. Pat. No. 6,856,640 (HENRICH et al.)). Although some references address the tailoring of pulse train sequences emitted from such systems (see U.S. patent application published under no 2006/0018349 (KOPF et al.)), most schemes relying on solid-state lasers provide very limited or no tuning of the pulse repetition rate and pulse duration. In addition, solid-state lasers lack the near diffraction-limited beam quality that sets apart fiber lasers and amplifiers from other types of laser sources.
A pulsed source with a tunable pulse duration and amplitude is disclosed in U.S. Pat. No. 7,813,389 (PENG et al). However, the minimum pulse width of this source cannot reach the picosecond regime as it is limited to 1 ns, with rise time between 1 ns and 3 ns and even longer fall time.
Another pulsed source with a tunable pulse duration is disclosed in U.S. Pat. No. 5,742,634 (RIEGER et al). However the disclosed source is a bulky free space laser, the pulse width range covered by this source is limited from 60 ps to 300 ps and the repetition rate is as low as 10 kHz, with no bursting capabilities.
There remains a need for fiber lasers able to produce pulses and bursts of pulses in the picosecond range and alleviating at least some of the drawbacks of the technologies described above. Such fiber lasers may be better suited to the requirements of micromachining and similar industrial applications. Incoherent sources able to produce pulses and bursts of pulses in the picosecond range are also of interest in other various applications, including gated illumination systems.